1. Field of the Invention
The present invention relates to navigating a moving body with the use of GPS signals and more particularly to the use of reflected GPS signals to passively navigate an orbiting body towards an orbiting target.
2. Description of the Related Art
Spacecraft require equipment that calculates the orbital position, velocity, time, and attitude. The advent of the Global Positioning System (“GPS”), increased the desirability of using onboard processing systems for spacecraft position determination.
The GPS satellite system is a collection of satellites that can be used for missile, satellite, aircraft, and terrestrial navigation. Each GPS satellite broadcasts its own ephemeris and time, thereby allowing a GPS receiver to determine its position. Typically, a GPS receiver calculates its position from the simultaneous observation of any four GPS satellites in order to calculate its position. A civil grade GPS receiver can accurately determine its position within 20 meters, determine its velocity within 0.6 meters/second, and the current time with a 10-nanosecond accuracy. At least twenty-four valid GPS satellites are in the GPS constellation at any given time.
A GPS receiver is an autonomous instrument that transforms signals from GPS satellites into point solutions for spacecraft navigation. Current GPS receivers have a radio frequency section for receiving and converting signals received from a spacecraft's antennas. The digitized signals are then forwarded to one or more correlators, controlled by the receivers own processor. The correlators look for matches between the incoming signal and the C/A code for different satellites. When a satellite lock occurs, or the incoming signal matches an internally generated pseudo random noise (PRN) code, the receiver's processor is notified. The processor contains executable code to generate a pseudo-range or a line-of-sight distance to the satellite. The processor may also contain the executable code of an orbit propagator. The pseudo-range is a measurement input to the navigation filter that calculates point solutions for determining the orbit of the spacecraft. While such GPS systems may be employed to position and track an orbiting moving body such as the Space Shuttle, such systems do not provide the ability to determine the position relative to another orbiting body. Conventional GPS receivers can not passively provide relative navigation or relative position data which are necessary during formation flying, docking, or other approach of space craft to proximity to another orbiting body.
The space environment poses additional obstacles to that of land based tracking systems. Because of the speeds associated with spacecraft motion, GPS satellites are in view for much shorter time periods than when viewed from a ground reference. The appearance and disappearance of GPS satellites from view causes their output signals to slew through the entire range of the 45-kilohertz Doppler shift. Typical space craft formation flying and autonomous rendezvous missions rely on active transmissions schemes such as RADAR (RAdio Detection and Ranging) and LIDAR (Light Detection and Ranging) which require specialized hardware requiring additional mass and power consumption. With the extreme weight and power restraints associated with space flight, there is a particular need for a reliable navigation/positioning system which reduces weight and power consumption requirements, as well as taking advantage of free GPS signals.
The use of Reflected GPS signals per se is also known. U.S. patent application publication 2006/0077094 entitled Information “Information Gathering Using reflected Signals” discloses a method of using reflected signals in a land based application to monitor the presence or absence of an object such as if a vehicle occupies a parking space and is hereby incorporated herein in its entirely. This system is not only ill suited for orbital applications, but does not make use of the reflected signal to navigate to a target.
It is also known to use Bistatic Radar Systems where a radar transmitter located remotely from the projectile (such as onboard a ship) illuminates the target and the reflected returns are received by a receiver located on the projectile. The tracking data from the radar measurements are then used to calculate the proper guidance signals to direct the projectile to the target. An example of a bistatic radar system is discloses ill U.S. Pat. No. 6,653,972 which in incorporated herein it is entirely. These systems require the use and control of an active radar transmitter and do not provide a suitable global positioning function. Rather, these systems are employed to simply acquire and hone in on a particular target. Therefore, such Bistatic radar systems are not suited for orbital positioning and navigation or have the ability to rely on a passive autonomous system.
One of the objects of the present invention is to utilize reflected GPS signals to provide passive autonomous relative navigation of an orbiting spacecraft towards an orbiting body.